Biological Control of Insect Pests on Field Crops in Kansas
J.P. Michaud, P.E. Sloderbeck, and J.R. Nechols
Biological Control beneficial practices can be integrated into production In the simplest terms, biological control is the systems in a cost-effective and convenient manner. This reduction of pest populations brought about through requires not only a good understanding of the biology the actions of other living organisms, often collectively and ecology of natural enemies, but also a willingness to referred to as natural enemies or beneficial species. modify production practices to accommodate their needs. Virtually all insect and mite pests have some natural Complicating matters is the fact that many beneficial enemies, although not all are effective in suppressing pest insects feed not only on pest insects, but also on each populations. Learning to recognize, manage, and conserve other, a phenomenon referred to as intraguild predation. natural enemies can help reduce pest populations and In a healthy agroecosystem with a high degree of plant maintain them below economic levels, thus reducing crop and insect diversity, naturally-occurring biological control losses and the need for more costly control measures that can go almost unnoticed because natural enemies are may also have undesirable environmental side-effects. effectively maintaining a plethora of potential pests at low Biological control is often most effective when densities. coupled with other pest control tactics in an integrated The most obvious harmful practice is the use of pest management (IPM) program. Practices that are insecticides at times when natural enemies are active. often compatible with biological control include cultural Insecticides have a wide range of adverse effects on controls, crop rotation, planting pest-resistant varieties, natural enemies, killing them directly, impairing their using insecticides with selective modes of action, or spot foraging and reproductive abilities, and depriving them of treatments that leave untreated areas to serve as refuges food. Nevertheless, there are various ways that insecticides for natural enemies. can be successfully integrated into a production system Effective biological control often requires a good while minimizing their impact on beneficial species. understanding of the biology of the pest and its natural Materials such as certain varieties of Bacillus enemies, as well as the ability to identify various life thuringensis (Bt) are selectively toxic to particular groups stages of relevant insects in the field. Field scouting of pest insects such as caterpillars and leave natural usually is necessary to monitor natural enemy activity, enemies unharmed. Unfortunately, most Bt formulations evaluate impact on pest populations, and anticipate the currently available lack good residual efficacy under need for additional control measures. Although three field conditions. The in-plant expression of Bt toxin in distinct approaches to biological control are recognized genetically-engineered corn and cotton varieties has (conservation, importation, and augmentation), the revolutionized the management of moth pests on these principles of conservation biological control are by far the crops and has so far proven compatible with biological most important for producers of field crops in Kansas to control of other pests, primarily through reduced understand. applications of hard insecticides. Conservation of Natural Enemies Several recently developed biorational materials such as spinosad and indoxacarb achieve selectivity through Natural enemy conservation is at once the most low contact toxicity; they must be consumed by the straightforward concept of biological control in the insect to be activated. Since natural enemies typically context of field crops, and the most complex. At the don’t eat plant material with the chemical on it, they are simplest level, conservation biological control means spared direct mortality. However, even materials with avoiding cultural practices that harm natural enemies high contact toxicity can be applied in ways that minimize and implementing practices that attract, encourage, or their impact on beneficial species. Insecticides with good benefit them. Although this may seem like common plant systemic activity (e.g., clothianidin, imidacloprid, sense, the challenging part is understanding exactly thiamethoxam) can be used as seed treatments, in-furrow, what practices are harmful to natural enemies and how 1 or as soil drench applications to be taken up by the harvesting practice provides a continuous refuge and food plants while causing little direct mortality to natural supply for beneficial insects. enemies. Finally, damaging pest populations are often Importation of Natural Enemies confined to portions of a field, rather than being evenly Today’s high volumes of international trade and distributed throughout it. Selective spot-treatment of airline traffic have increased the frequency of arrival affected areas will not only reduce application costs, it of exotic pests from other regions of the world. When will leave untreated areas to serve as reservoirs for natural immigrant pests succeed in establishing in a new enemies. These survivors can then recolonize treated areas geographic location they can rapidly reach very high following degradation of the insecticide, accelerating populations and cause serious economic losses, largely the restoration of biological control and sometimes because they lack the complex of natural enemies that averting the need for subsequent treatments. Nevertheless, limits their population growth in their country of origin. situations arise where the preservation of biological Examples of serious pests in Kansas of foreign origin control requires avoiding the broadcast application of any include the Hessian fly, the European corn borer, the insecticide. When biological control is widely disrupted Russian wheat aphid, and the alfalfa weevil. The selective by insecticide applications, minor pests can become major importation and release of natural enemies from the pest’s pests (secondary pest resurgence), and a farm can develop country of origin is also known as classical biological dependency on chemical control measures once natural control. This approach gained impetus early in the last enemies are no longer resident in the fields (the pesticide century following several dramatic successes, notably treadmill effect). the importation of the vedalia beetle to control cottony Certain cultural practices also can be detrimental cushion scale in California. However, only a small to natural enemies. Plowing, cultivation, mowing, or proportion of importations have met with this level of harvesting operations can be disruptive to natural enemies success and the general applicability of this approach to if they coincide with critical stages of their life cycle. all new invasive pests is currently a subject of considerable While the adoption of no-till and minimum tillage debate. Although large populations are often observed agriculture has favored the resurgence of some pests such when an exotic pest first invades, these often decline as false wireworms that utilize crop residues for food or over a period of years as the native community of natural harborage, such practices also favor various beneficial enemies gradually evolves to exploit them. Evidence is insects such as ground beetles, spiders, and other growing that many invasive pests ultimately come under generalist predators that rely on crop residues for cover, adequate biological control solely through the action which encourages their persistence in the field. Dust of native beneficial species. However, there are other raised by traffic along dirt roads or cultivation operations examples where native natural enemies do not — or carried out during dry weather can impede the foraging cannot — provide required levels of biological control. activities of beneficial insects when it is deposited in Classical biological control involves exploring a sufficient amounts on leaf surfaces. The burning of crop pest’s country of origin for a potentially effective natural residues can also kill large numbers of beneficial insects, as enemy, importing it to the pest’s adopted country, can inappropriately timed flood irrigation. Other practices and mass-rearing it in the laboratory for subsequent such as excessive herbicide applications on pastures and release in regions where the pest is active. The goal is fallow fields reduce both plant and insect diversity in non- to ultimately establish a self-sustaining population of crop habitats that normally serve as important reservoirs the natural enemy that maintains the pest population of many natural enemy species. These beneficial species below economic threshold levels for perpetuity. In this are then no longer available to colonize annual crops in regard, classical biological control differs from other adjacent fields as pest populations develop. forms of biological control in that it is not carried out One of the biggest hurdles for sustainable biological by the farmer or gardener, but only by scientists with control in field crops is the loss of plant and insect appropriate authorization from federal agencies, in diversity associated with large scale monoculture, particular the United States Department of Agriculture. a configuration that tends to favor pests over their Non-native insects must be held under strict quarantine natural enemies. Conservation of natural enemies can conditions until it can be ascertained that (1) they have be generally improved by avoiding a completely clean some potential to control the target pest, (2) they will farming approach and by taking steps to encourage plant not themselves become pests, and (3) they do not harbor and insect diversity in non-cultivated areas and wherever their own natural enemies that might interfere with else this may be feasible. Strip-harvesting of alfalfa, their effectiveness or that of other beneficial insects. In where alternating rows of alfalfa are left uncut until the addition, prospective natural enemies are evaluated to cut areas begin to grow back, is an excellent example of determine their potential to attack and/or feed on other conservation biological control because this modified beneficial species. Growing awareness and concern about potential non-target effects of released natural enemies inoculative. Inundation involves releasing large numbers has led to increasingly stringent criteria for introductions. of natural enemies for immediate reduction of a damaging These restrictions are justified given that other, far or near-damaging pest population. This strategy is used more complex, ecological impacts of introduced species mainly for short-term control and is only feasible for are exceedingly difficult to predict. A good example is natural enemies that can be produced in large numbers the multicolored Asian lady beetle, Harmonia axyridis. very cheaply. It is applied as an immediate, corrective Although this species is an excellent biological control measure; successful reproduction and continued survival agent and a voracious predator of aphid species on many of the natural enemy population is neither presumed nor crops, it appears to have displaced many native lady expected. Inundative releases are often used as a substitute beetles from their habitats and become a serious urban for a chemical spray that might be undesirable because of pest in many regions. unwanted side-effects, hence the term biopesticide. In Augmentation of Natural Enemies contrast, inoculation involves releasing small numbers of natural enemies once or more throughout the period of To some people, biological control means buying and pest activity, usually beginning when the pest is still at low releasing natural enemies to control pests. This approach density. It is therefore more of a preventative measure, in is generally known as augmentation, although in the that the natural enemy is expected to reproduce and build strictest sense, augmentation refers to situations whereby a population that will prevent the pest from reaching the natural enemies are released to supplement an existing economic injury level. population, something that is rarely done in field crops. Augmentative biological control is only reliably Augmentative biological control is widely recognized by effective when it rests on a solid foundation of research the public because many commercially available insects derived from the specific context of application. Although are advertised for sale in magazines and publicized in many purveyors of natural enemies recognize the need the media. Further, the use of pesticides has conditioned to provide an insect + information package to their us to think about pest management in the context of clientele to maximize the chances for success, many others purchased products. However, in contrast to the other are primarily concerned with selling insects, with the two forms of biological control, augmentation is less result that many are sold for inappropriate applications, sustainable because it relies on regular or periodic releases sometimes generating consumer dissatisfaction with of purchased products, something that sometimes benefits biological control in general. A prime example is the producers of these products more than consumers. sale of adults of the lady beetle, Hippodamia convergens, Situations do exist where augmentation can be highly to control aphids. Dozens of outlets offer these insects efficacious, cost effective, and a desirable alternative to for sale, including some of the largest agricultural pesticide applications, but these occur mostly in enclosed supply companies, despite general agreement among settings such as greenhouses and interior plantscapes. entomologists that they are not effective in this Natural enemy augmentation is based on the application. Most commonly, the adult beetles are assumption that, in some situations, there are not collected in buckets en masse from overwintering adequate numbers of natural enemies to provide sufficient aggregations in the mountains of California. This practice biological control (even though some may be present), is fraught by at least two problems. As they emerge from or that their immigration is not timely enough to hibernation, the beetles instinctively disperse rather than suppress the pest before it reaches economic levels. One feed and lay eggs. Consequently, most fly away from requirement for augmentation to be feasible is a source their release sites immediately, regardless of the presence of large numbers of natural enemies that are readily of aphids. Secondly, beetles collected from hibernation available for an affordable price. In response to demand, are typically in reproductive diapause, a dormant state in many companies have developed insectaries capable which eggs are not laid and adult feeding may be minimal, of producing large numbers of predatory and parasitic resulting in poor performance. A general problem, not insects and others produce and market nematodes and specific to lady beetles, is that populations reared in the microbial pathogens as other forms of biological insect insectary, or field-collected from certain localities, may be control. Unfortunately, while these companies may have adapted to specific environmental conditions substantially developed good techniques for rearing and disseminating different from those where they are released. If local their beneficial insects, they typically have limited climate or food sources are considerably different from experience with regard to how, where and when to release those to which the populations is adapted, survival and these insects into the crop for greatest impact. Likewise, reproduction may be greatly diminished at release sites. only rarely do recommendations identify circumstances Even when an appropriate natural enemy is selected, when releases of natural enemies should not be made. failures to achieve satisfactory control can result from There are two general release strategies in a number of causes, most commonly a user’s lack of augmentative biological control: inundative and information on the biological requirements of the insect, It can provide a safe alternative for controlling certain the biology of the target pest, and effective modes of pests in some situations, but a significant investment in application. Another problem is that appropriate release research is required for it to provide reliable control in any rates are difficult to determine for a given situation and given situation. It is the responsibility of the end user to depend on many factors, not least of which is the actual obtain and assimilate the relevant information necessary pest density at the time of release. The best advice for pest for effective implementation of a release program and managers considering an augmentation approach is to ensure that the product purchased is appropriate for the do their own research and obtain as much information particular pest and situation. There are no commercially as possible to maximize the probability of success. Well- available natural enemies that are currently recommended researched projects can and do result in very effective for augmentative biological control applications in large- augmentative biological control programs, including those scale commercial production of field crops in Kansas. involving applications of microbial insecticides. Recognition of Common Beneficial Insects in Kansas The cost of commercially supplied natural enemies Field Crops is a major consideration in assessing their potential Predators suitability as an alternative for pesticides in any situation. By predators we refer to insects or other arthropods Prices vary widely because of differences in the degree (spiders and mites) that feed on other insects (prey) by of difficulty and expense in rearing different species. hunting, killing and directly consuming them. Although Sometimes it is justifiable to pay a higher cost for natural more than 100 families of insects contain predaceous enemies relative to an insecticide, provided adequate species, about 12 of these contain the major biological control of the pest is obtained. This may be the case control agents of field crop pests. Here we summarize the where pests have developed insecticide resistance, where biology and description of some of the most important worker protection standards are a concern, where there are families. risks of disrupting biological control programs for other pests with insecticides, or where a premium price can Lady Beetles (Coleoptera: Coccinellidae) be obtained for certified organic production. In general, Lady beetles, or ladybugs, are possibly the most the inoculative approach is more cost effective provided universally recognized group of beneficial insects. They that correct release rates and timing are used. Inundative are found almost anywhere and usually feed on aphids releases can be justified on high-value crops where the and a variety of other soft-bodied insects. Some will cost of production is already substantial. Managers should also feed on mites and the eggs and larvae of moths and carefully evaluate the cost of a natural enemy as they beetles. Currently there are about five species that can be would any other production cost before making a decision commonly encountered in field crops in Kansas. on augmentative release. Probably the most abundant species in Kansas field Important questions to ask before considering an crops is the convergent lady beetle, Hippodamia convergens, augmentation program: a native species. This lady beetle is recognizable by the two convergent white lines on the black portion of the body 1. Has research shown that a release program can be immediately behind the head (Fig.1). Coloration ranges effective for a particular pest, crop and local situation? from pale orange to red with a series of black spots that 2. What is the best time to release the natural enemy in may only be faintly visible, or entirely absent. Aphids are relation to the pest’s life cycle? the preferred food, although adults may supplement their 3. Are releases compatible with other crop production diet with other prey items and certain vegetable food practices that are anticipated, including the possible need to apply pesticides against other pests? 4. Does the supplier provide a comprehensive information package with clear instructions on handling, releasing, and evaluating the effectiveness of the natural enemy? 5. What quality control practices does the supplier use to ensure that insects arrive alive and in good condition? 6. How does the overall cost of a release program compare with alternative control strategies when all ancillary costs and benefits are factored in? To summarize, augmentation is perhaps the most publicly recognized form of biological control, but also one of the least understood and most often misapplied. Fig. 1
sources. Overwintered adults produce one generation in corn borers. This species is often abundant in proximity early spring that is typically completed around the time to water sources such as rivers and lakes. It hibernates wheat is harvested, leading to the mass exodus of large in aggregations during winter months but, unlike the numbers of beetles from maturing wheat fields. First- convergent lady beetle, probably continues to reproduce generation females mate within days of emergence, but throughout summer months, typically producing 3 or 4 produce very few mature eggs right away unless they are generations per year. able to encounter a large supply of aphids. Rather, the A species becoming increasingly common in majority produce fat bodies to store the energy gleaned the Midwest with the advent of soybean aphid is the from whatever prey they find and forgo reproduction multicolored Asian lady beetle, Harmonia axyridis (Fig. for the duration of summer, surviving dry periods by 3). Adults are highly variable in coloration (pale orange drinking the sap from various plant species, especially to red) and spotting patterns (males often have none). sunflower. As fall brings cooler weather, aphids once The key to identifying this species is the prominent again increase in abundance and the over-summered black ‘W’ on the part of the body behind the head. This females lay their bright yellow or orange eggs in clusters near aphid colonies to begin the next generation. The number of generations is variable, depending on the food supply, but the majority of adults maturing late in the season delay reproduction and conserve their resources for hibernation. They crawl into protected sites, typically at the base of grass tussocks, and remain dormant through winter months until they are awakened by the warm temperatures of spring. These abilities to hibernate in winter and forgo reproduction during summer when prey are scarce represent adaptations that are likely key to this species’ success and abundance in prairie habitats. Another common species is the twelve-spotted or pink lady beetle, Coleomegilla maculata. The adults are pink Fig. with six black spots on each wing cover (Fig. 2). Aphids are also the preferred food of this species, but a very wide range of insect prey may be consumed. The larvae of this species are unique among lady beetles in their ability to complete development on an exclusive diet of pollen.
invasive species is a voracious predator of aphids and many other insects, including the larvae of alfalfa weevil. Unfortunately, it can also complete development feeding on the eggs and larvae of many other lady beetles and has been implicated in the declining abundance of a number of native species, a fact that has marred its reputation as an otherwise effective biological control agent in many types of agricultural production. Another habit contributing to the potential pest status of this beetle is a propensity for entering houses in fall and winter, often forming large aggregations that can be very distressing to homeowners. The seven spotted lady beetle, Co ccinella Fig. septempunctata, is another imported species, one that originates in Europe. It is a large beetle that can be recognized by the distinctive seventh black spot that spans Pollen is also attractive to adults and many can be found the front edge of both wing covers and is flanked by two in corn fields at tasseling, although they also like to feed small white triangles (Fig. 4). It prefers to feed on aphids on the eggs and small larvae of many moth species that infesting grasses and herbaceous plants. are abundant in corn, including economically important
Fig.
Fig.
Other, smaller species of ladybeetles can also be abundant in Kansas fields, but may go unnoticed because of their small size and more secretive habits. Many are important for feeding on the eggs of moth pests such as the European corn borer. Scymnus spp. (Fig. 5) have larvae that produce waxy secretions that serve to defend them against ants, causing them to resemble mealy bugs
Fig. 7
Fig.
(Fig. 6). Others such as Stethorus spp. are even smaller and specialize in feeding on mites (Fig. 7). Most aphid-feeding species lay bright yellow or orange eggs in clusters of 15 to 40, usually near, but not directly in, aphid colonies. The larvae of ladybeetles are Fig. 8 more difficult to identify to species than adults, but larvae of most of the larger species resemble little alligators. There are four larval instars (molts) before the pupal stage Hover Flies (Diptera: Syrphidae) is reached. Hover flies (Fig. 8) are also known as flower flies and are most easily recognized by their hovering flight above flowers or aphid-infested plants. Many resemble bees or wasps, but different species vary greatly in size and by the larva in its period of development. The pupae are appearance. The larvae (Fig. 9) are voracious predators of typically teardrop-shaped (Fig. 11) and may form on plant aphids and, in some cases, other small, soft-bodied insects. parts below the aphid colony, or in the soil, depending on
Fig. 9 Fig. 11
Adult hover flies require access to flowers as they need sugar from nectar to fuel their flight. In addition, species. The pupa is also the overwintering stage. Syrphid female flies must consume pollen as a source of protein flies comprise an important, if often overlooked, source before they can mature their eggs. Adult females find of aphid mortality that, acting in concert with other aphid colonies by orienting to the smell of honeydew predators and parasitoids, is important in keeping aphid excreted by the aphids. Many studies have shown that populations below damaging levels. planting mixed borders of wildflowers around gardens Lacewings (Neuroptera: Chrysopidae and can attract hover flies and increase the rate of egg-laying Hemerobiidae) on aphid colonies on adjacent vegetables and other crops. All lacewings are predaceous as larvae, and adults of This is an example of conservation biological control by some species are predaceous as well. Lacewing larvae (Fig. habitat management and is the only effective means of 12) prefer aphids as prey but also consume a range of encouraging these insects. other soft-bodied pests such as mites, thrips, leafhoppers Different species of hover fly vary in the kinds of and mealybugs. The most common species in Kansas field aphid colonies they select for laying their eggs and many are quite specific to certain aphid species. The white, oblong eggs (Fig. 10) are usually laid singly in among the
Fig. 10 Fig. 1
crops is the green lacewing, Chrysoperla carnea, although aphids. A maggot hatches from the egg in 2 or 3 days and other species also occur. Adults have large, lacey wings, begins to feed on aphids voraciously, growing at a truly thread-like antennae, and protruding eyes (Fig. 13). They remarkable rate. The larvae are slug-like, tapered toward are primarily nocturnal, but when disturbed, they will the head, and use a film of their own saliva to adhere leave their resting places on the undersides of leaves in an to the leaf surface. There are typically three instars that erratic, fluttering flight. Despite their fragile appearance, may be completed within 7 to 14 days, depending on lacewings are among the very few insects capable of temperature, and as many as 400 aphids may be consumed extricating themselves from a spider’s web. The white, 7 Fig. 1
contents like soup through a straw. This process is known Fig. 1 as extraoral digestion (which is also used by lacewing larvae). A wide range of prey are taken depending on the size and species of bug. Minute pirate bugs are partial to thrips, but other bugs will feed on aphids, caterpillars, and insect eggs. The larger assassin bugs (Reduviidae) are top predators in the food chain and feed on many kinds of insects, even lady beetles. This is an example of intraguild predation – predators eating each other as well as feeding on pest species, a phenomenon that often adds complexity to biological control systems. Ground Beetles (Coleoptera: Carabidae) Ground beetles are commonly found in all cultivated field crops. Both larvae and adults are predaceous on many Fig. 1 ground-dwelling insects, but their actual contribution to biological control of crop pests is not well understood. Most species are large, shiny and black, with ridged wing oval eggs are laid on the end of long stalks (Fig. 14). The covers. They have threadlike antennae and a head that is relative length of the stalk and the pattern in which the smaller than their thorax. Adults of most species rarely fly eggs are laid (singly versus in groups, in line or in a spiral) and are most often seen running across the soil surface. can be characteristic of particular species. Most adult feeding occurs on the soil surface, and most Lacewings are among the beneficial insects that are larval feeding under the soil surface, so they feed on root available commercially. Usually, the eggs are shipped maggots, rootworms, caterpillars, and other soft-bodied mixed with a substrate such as rice hulls and some moth insects that might be dislodged from plants. For example, eggs for food. However, because the larvae are highly ground beetles have been studied for their contribution cannibalistic and immediately begin to kill and eat each to biological control of cereal aphids, even though most other upon hatching, they require immediate distribution will not climb a plant to reach the aphids. However, as in the field. Even then, lack of biological information on lady beetles and other predators attack an aphid colony, habitat preferences, climatic tolerances, dormancy, and they often dislodge more aphids than they actually eat, the behavioral responses of particular species, as well and carabid beetles foraging on the soil surface reap the appropriate release rates and suitable release techniques, rewards, and ensure that these aphids are not able to have severely limited the usefulness of these insects in climb back onto the plants. augmentative biological control programs. Spiders True Bugs (Hemiptera: Anthocoridae, Nabidae) Although spiders are not insects, they play an Numerous species of ‘true’ bugs are predators of insect important role as generalist predators of many insect pests. These include damsel bugs, (Nabis spp., Fig. 15), big- groups. As such, their role as biological control agents is eyed bugs (Geocoris spp.), minute pirate bugs (Orius spp.) perhaps greater than previously thought. Their presence and assassin bugs. Predatory bugs skewer their prey with also is considered by many to be indicative of a healthy piercing and sucking mouthparts, inject enzymes to digest agroecosystem. Spiders comprise a very diverse group that the internal organs, and then drink the liquefied body can be broadly categorized by their hunting strategies. Web-spinners, including orb-weavers, and garden spiders, 8 use silk to trap their prey in various ways. Species such as controlling the fertilization of eggs; males are produced crab spiders are highly cryptic ‘sit and wait’ predators that from unfertilized eggs and females from fertilized hide in flowers to ambush pollinators. Hunting spiders are ones. In some species, all-female lines persist for many typically hairy, robust species that do not build webs, but generations without sexual reproduction. The female uses actively seek out their prey. These include jumping spiders, an ovipositor to lay eggs in a host insect (the stinger of wolf spiders and the large tarantulas. a honey bee is a modified ovipositor that delivers only Parasitoids venom). In some species the ovipositor is held internally when not in use; in others it is not retractable and may Parasitoids are like the vampires of the insect world. be as long, or longer than, the entire wasp body. Venom As immatures, they obtain their nutrition by feeding in that serves to immobilize, paralyze or otherwise subdue or on the body of another insect, ultimately killing it. the host may also be delivered via the ovipositor. Some The adults are typically free-living and the females are female parasitoids also use the ovipositor to puncture a responsible for finding host insects for their progeny. The host and then feed on the body fluids before selecting two major groups discussed here are parasitic wasps and other hosts for oviposition, thus causing two different tachinid flies. types of mortality in the pest population. In some cases, Parasitic Wasps the egg is laid externally on the body of the host and the Parasitoid wasps comprise one of the most diverse larvae may also feed externally (ectoparasitism). More and important groups of beneficial insects. Almost all commonly, the larva develops and pupates within the host insects are attacked by at least one species of parasitoid, body, feeding selectively on the host’s internal tissues and and most by more than one. Some species attack only leaving the digestive tract and nervous system to the very one insect host and many successful classical biological last (endoparasitism). Another important distinction is control programs have involved the introduction of highly whether the host is allowed to develop and grow with specific parasitoids. Many species are large and colorful, the parasitoid larva inside it, or whether it is killed or but most of the economically important ones are small permanently paralyzed by the attacking females so that it and very inconspicuous. For example, those attacking remains a static, rather than dynamic, food source for the aphids are smaller than their aphid hosts (Fig. 16), and developing larva. those developing within a single moth egg or scale insect Tachinid Flies Fig. 1 This group represents a very large family of flies with more than 1,000 species in North America, all of which have a parasitic lifestyle. They vary considerably in appearance, but most have bristled bodies and resemble house flies, although they can be substantially larger or smaller (Fig. 17). The adult female typically lays an egg on the surface of the host insect cuticle, and the hatching
Fig. 17
are barely visible to the naked eye. Some are solitary, with only a single individual completing development in a host insect, whereas others are gregarious, with as many as several hundred siblings feeding and developing on the same host. This condition most often results from the female laying many eggs on the same host, but in some species a single egg undergoes a series of divisions before development begins, a condition known as polyembryony. larva then bores into the body of the host and develops Parasitoid biology is distinct from that of many internally. In other cases, the fly egg is consumed by the other insects. Although reproduction is typically sexual, host insect while it feeds. Some species give birth to live most females can manipulate the sex of their progeny by larvae that are placed directly onto the host. A wide range 9 of moth and butterfly larvae are attacked, and so are a Conclusions number of beetle species. The host may be killed in the Biological control is a natural process that plays an adult stage, but more commonly in the pupal stage. important role in the suppression of field crop pests in Nematodes Kansas. A lot of the best examples of biological control proceed completely unnoticed by the farmer, simply Nematodes are a phylum of roundworms that are because both the pest and its natural enemies coexist at among the most abundant multicellular organisms on such low densities that there is no perceptible problem earth. Although many families of nematodes feed on in the crop, even thought the pest is present. This fact plants and include many important pest species, some has been demonstrated by using pesticide treatments to are free-living whereas others are obligate parasitoids of disrupt natural enemy populations and then observing insects and include many important biological control formerly insignificant insect populations rise to the status agents. Some are produced and sold commercially for of major pests. It is generally agreed that integrated control of soil and foliar insects. Nematodes are normally pest management (IPM) is the preferred approach to applied as either a spray suspension or a soil drench, but sustainable pest control in agriculture. Whenever possible, their survival and efficacy is often dependent on soil type IPM programs for field crops should be constructed on a and adequate moisture. They are associated with a number foundation of biological control, with additional control of different commensal and symbiotic bacteria that aid measures applied only as needed and selected to conserve them in killing and digesting their host insects. natural enemies and all non-target insects to the greatest Microbial Pathogens extent possible. The best way a farmer can improve his A variety of microbial pathogens, including bacteria, chances of benefiting from biological control (e.g. saving protozoans, viruses and fungi are specifically pathogenic the expense and hazard of pesticide applications) is by to insects and completely harmless to other forms of learning to recognize those beneficial insects that are life. This selective pathogenicity renders many of them important natural enemies of the key pests attacking his valuable as biological control agents of insect pests. crops. These insects should be thought of as farm laborers Some, such as Bacillus thuringensis, have been the source that work for free — the only wages they demand are the of natural insect toxins that are now synthesized as pests that they consume. While more research is needed biopesticides, or engineered directly into crop plants. to better understand the needs and requirements of many Many insect diseases caused by pathogens are very beneficial species so that their effectiveness might be important sources of mortality in pest populations enhanced, there are many recognized techniques available and lead to precipitous population declines when to generally conserve natural enemies and encourage their they become epizootic (analogous to an epidemic in a activities. The advent of no-till agriculture has likely had a human population). However, many attempts to induce net positive effect on the abundance of beneficial insects epizootics in pest populations by means of distributing in field crops. Crops genetically modified to express spores or other types of inoculum fail because stringent natural insect toxins have, to date, proved either neutral environmental conditions are often necessary for or favorable to biological control by virtue of reducing successful infection and/or transmission of the disease. overall pesticide usage. Fallow areas around fields can For example, many fungal diseases of insects require serve as reservoirs of many natural enemies, especially if high humidity or prolonged leaf wetness in combination weed species are allowed to flower. If a pesticide treatment with particular temperatures to infect their hosts. becomes necessary, leaving less-affected portions of a field Some success has been obtained with baculoviruses untreated can provide a refuge for natural enemies and commercially formulated with sunscreens to protect them accelerate the field-wide restoration of biological control from solar radiation when sprayed onto plant surfaces. post-treatment. Some newer pesticide formulations are However, most insect epizootics proceed without human designed to be more selective for particular pests and assistance when suitable environmental conditions arise. will spare natural enemies. In short, whenever biological Conservation is also a consideration when epizootics are control has a role in pest population reduction, pest a significant natural mortality factor in pest populations. control decisions should be weighted by considerations of For example, excessive use of fungicides to control how natural enemies will be impacted, and what tactics powdery mildew and other foliar diseases in potatoes can also eliminate insect-pathogenic fungi, thus favoring aphid outbreaks.
10 might be feasible to conserve them. Other Resources Insect Biocontrol Laboratory U.S. Department of Agriculture: http://www.ba.ars.usda.gov/psi/ibl/ Biological Control Information Center, University of North Carolina: http://cipm.ncsu.edu/ent/biocontrol/ Biological Control: A Guide to Natural Enemies in North America: http://www.nysaes.cornell.edu/ent/ biocontrol/ Nematodes as Biological Control Agents of Insects: http://nematode.unl.edu/wormepns.htm The Association of Natural Biocontrol Producers: http://www.anbp.org/ Photo Credits Figs. 1,2,3,4,5 and 13: J.P. Michaud. Figs.14 and 15: P.E. Sloderbeck. Figs. 7 and 16: U. Wyss. Fig. 8: Georgia Museum of Natural History. Fig. 6: University of Idaho. Fig. 12: E. Prado. Figs. 9,10,11 and 17: unknown.
11 J.P. Michaud, P.E. Sloderbeck, and J.R. Nechols, entomologists Kansas State University
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